Cellular radio systems are currently in widespread use throughout the world providing telecommunications to mobile users and to fixed wireless access subscribers. In order to meet the capacity demand, within the available frequency band allocation, cellular radio systems divide a geographic area to be covered into cells. A base station communicates with the mobile users and to fixed wireless access subscribers over the air interface, each base station typically being equipped with directional antenna arrays arranged in three or six sectored sub-cells where the higher gain of the narrow beamwidth antennas improve the uplink from the lower power mobiles. The distance between the cells is determined such that co-channel interference is maintained at a tolerable level.
Each base station has a connection with a base station controller and is typically linked with a standard (wired) public telephone service to provide links with ordinary telephone subscribers. A base station antenna arrangement typically comprises a number of antennas, such as, dipole, patch or layered antennas. The antennas are contained within an antenna enclosure. The dimensions of an enclosure in the range 0.1-3 m wide, with the height typically being of a similar order. Antenna enclosures of the so-called smart variety also contain beamforming electronics within the enclosure.
Whilst the internal space within the antenna is ideally hermetically sealed, this is not possible due to, inter alia, maintenance requirements and the physical characteristics of the different components within the antenna, such as differential expansion which can be as great as 10 mm across the length of an enclosure. The antenna radome (the part of the antenna through which the antenna radiates) must be transparent to microwave radiation and can be of a generally light weight construction, whilst the back to the enclosure must be sufficiently strong to provide support for the antennas themselves, electronics (if installed and allow the whole antenna to be mounted upon a mast or similar support, and is accordingly not insignificant in weight--the weight of an enclosure can be up to 150 kg or so. Even if it was possible to totally seal an enclosure, differences in temperature and pressure would distort the radome, which in turn would affect the microwave performance of the antenna. In practice, antenna enclosures are semi-sealed.
In order to provide a sufficient field of coverage, the antenna should be placed at a height above ground level, typically upon a building or a mast, extending 15 m or more. In such a position, the antenna enclosure is subject to extremes of temperature, windage and moisture. It has been determined that certain equipment such as antenna enclosures are susceptible to water ingress, which can accumulate at the inside bottom of such an enclosure. Such water ingress does not necessarily arise from precipitation; in areas with a high humidity and/or high diurnal temperature change can suffer severe condensation problems. With an antenna such as is shown in FIG. 1, in the case that only one antenna element was partially submerged by water, because of the layout of the antenna, this would have the effect that the lower four elements do not operate effectively, with a total reduction of around 1 dB being typical. Needless to say, this has a concomitant effect upon the reliability of the antenna. In the case of smart antennas, the associated electronics equipment may be susceptible to failure if subjected to moisture.
Presently, antennas are typically equipped with Gore-tex patches to enable water vapour to pass from the inside to the outside of the enclosure, but such patches do not allow water flow. A further approach has required the components within an enclosure to be coated with a protective finish and to provide relatively large holes for most water to drain out, but this is somewhat crude and maintenance and repair operations can be hampered and, further, such an approach is unsuitable for smart antennas which incorporate active electronics within the enclosure. Ventilators as used and developed for telecommunications control boxes and equipment racking, having a gauze or grid which allows air or liquids to filter through have been employed, but allow liquids to pass in both directions through the filter. A U-bend arrangement has also been employed, which requires the presence of a liquid--which cannot be guaranteed in warm conditions, also the liquid may get blown from the tube under positive pressure as the enclosure "breathes". Further, neither of these devices are truly non-return valves.
For a valve to operate in such a fashion that the atmosphere within the enclosure is controlled, then the amount of water present within the enclosure must be minimal, liquid must be able to pass through the valve at high rates, the size of any openings must be less than approximately 100 .mu.m, whereby insects, dust, pollen or other foreign bodies are prevented from entering the enclosure. Any valve mechanism must be able to self-clear, or, preferably, not get clogged under any circumstances. The materials of any components must be able to withstand climatic extremes and u.v. degradation.